Vitamin D insufficiency (variously defined as <20 to 30 ng/mL of total serum 25(OH)D as currently measured) is highly prevalent, even in otherwise healthy individuals. Reported in >1 billion people worldwide, it is now recognized as one of the most common subclinical medical conditions in the world. Beyond rickets, a manifestation of severe vitamin D insufficiency recognized since the 17th century,35 vitamin D deficiency (commonly defined as <10-25 ng/mL of total serum 25[OH]D)21,34,36 has since been associated with an increased risk of osteoporosis, cancer, infectious disease, CVD disease, allergy, asthma, multiple sclerosis, muscle weakness, rheumatoid arthritis, and diabetes. Low vitamin D can arise from insufficient intake from nutritional sources, insufficient synthesis (via UV-B radiation of the skin), adiposity, age, physical activity, or other disease-related factors such as diabetes, bariatric surgery, fat malabsorption syndromes, and kidney disease.14,37,38 The use of sunscreen with sun protection factor (SPF)≧30 reduces the ability of the skin to produce vitamin D by 99%, thus contributing to the pandemic.
One recent study found that vitamin D insufficiency was present in 72% of community-dwelling men older than 65 years of age, and in up to 86% of those men who were obese, lived at higher latitudes, or infrequently participated in outdoor activities.17 Although vitamin D deficiency is less common, it is estimated to affect 26%-54% of community-dwelling older adults and 57% of hospitalized patients.17,36,40 A recognized problem in older adults, people of all ages who live in diverse geographic locations are also susceptible, including sunny climate dwellers.41 A study of younger adults with limited exposure to the outdoors in a northeastern urban setting reported that 32% of students and doctors aged 18-29 years were vitamin D deficient at the end of the winter.42 In diseases including diabetes, rheumatoid arthritis, renal disease, as well as in individuals who are obese, hospitalized, pregnant, newborn, highly deficient levels of this hormone are common.40,43-45 Current recommendations for vitamin D supplementation are largely inadequate.17,46,47 According to Holick,46 25(OH)D is the most-ordered hormone assay in the US, used as the basis for treatment recommendations. However, assay results as well as cutoff levels of 25(OH)D to define the extent of vitamin D insufficiency are subject to considerable variation. Given the prevalence and breadth of illnesses potentially associated with low vitamin D, gaining a better understanding of vitamin D status to guide management of vitamin D insufficiency is a public health priority. The Institute of Medicine (IOM) has recognized that that assay variation and lack of consensus regarding cutoffs defining insufficiency/deficiency have caused confusion about the appropriate dietary intake of vitamin D.33 The IOM has also cautioned against excessive intake due to the risks of kidney and tissue damage and have urged more targeted research in this area. Importantly, the method used to determine vitamin D status should be clinically relevant and applicable across diverse populations.